At present, a water absorbent resin (a water absorbing agent) such as a cross-linked polymer of a polyacrylate salt is widely used, aiming at absorbing body fluid, as a constitution material of a hygienic article such as a disposable diaper or a sanitary napkin, so-called an incontinence pad or the like.
Such a water absorbent resin is required to have excellent property such as excellent fluid absorption amount or water absorption rate, gel strength, gel fluid permeability in contacting with aqueous fluid such as body fluid or the like, or excellent water suction force from a substrate containing aqueous fluid. Furthermore, in recent essential requirements include a water absorbent resin powder having very narrow particle size distribution, or a water absorbent resin powder having high absorption capacity or low extractables, and also high absorption capacity against pressure or fluid permeability against pressure.
U.S. Pat. Re No. 32649 has proposed a water absorbent resin excellent in gel strength, extractables and water absorption capacity; GBP No. 2267094B has proposed a water absorbent resin excellent in fluid permeability without load, water absorption rate and absorption capacity; technology specifying specific particle size distribution has been proposed in U.S. Pat. Nos. 5,051,259, 5,419,956, 6,087,002, EP No. 0629441 or the like. In addition, there are many proposals on water absorbent resins excellent in absorption capacity against pressure under various loads, or a measurement method therefor, and water absorbent resins with excellent absorption capacity against pressure only or other properties in combination have been proposed in EP No. 0707603, EP No. 0712659, EP No. 1029886, U.S. Pat. Nos. 5,462,972, 5,453,323, 5,797,893, 6,127,454, 6,184,433, 6,297,335 and U.S. Pat. Re No. 37021 or the like.
Among these properties, particle size has strong effect on other properties of a water absorbent resin, therefore, many methods for controlling particle size have been proposed. For example, as a method for controlling particle size, a method for recovering only a fine powder by separation, agglomeration or gelling has been proposed in U.S. Pat. Nos. 6,228,930, 4,950,692 (issued on Aug. 21, 1990), U.S. Pat. No. 4,970,267 (issued on Nov. 13, 1990), U.S. Pat. No. 5,064,582 (issued on Nov. 12, 1991) or the like; a method for polymerization of a powder of a water absorbent resin as a monomer has been proposed in U.S. Pat. Nos. 5,432,899, 5,455,284, 6,867,269 or the like; and a method for agglomeration of all of water absorbent resins has been proposed in U.S. Pat. Nos. 4,734,478 and 5,369,148 or the like.
In addition, also a method for a polymerization or gel crushing to maintain high productivity and property has been proposed in U.S. Pat. No. 6,906,159, US-2004-092688A, US-2004-234607A or the like.
However, trying to enhance property (for example, particle size control, enhancement of absorption capacity, reduction of water-extractables) of a water absorbent resin by technique proposed in U.S. Pat. Re No. 32649, GBP No. 2267094B, U.S. Pat. Nos. 5,051,259, 5,419,956, 6,087,002, EP No. 0629441, EP No. 0707603, EP No. 0712659, EP No. 1029886, U.S. Pat. Nos. 5,462,972, 5,453,323, 5,797,893, 6,127,454, 6,184,433, 6,297,335 and U.S. Pat. Re No. 37021, accompanies, in many cases, reduction of productivity or use of sub-raw materials, and thus increase in production cost of a water absorbent resin is not desirable, in the viewpoint that a water absorbent resin is a disposable raw material of a diaper or the like.
Methods for controlling particle size proposed in U.S. Pat. Nos. 6,228,930, 4,950,692, 4,970,267, 5,064,582, 5,432,899, 5,455,284, 6,867,269, 4,734,478 and 5,369,148 provide still insufficient control of particle size, and further addition of the new particle size controlling step (agglomeration or fine powder recovery) not only accompanied cost increase, but also provided lowering of properties other than particle size, depending on cases. In addition, also in U.S. Pat. No. 6,906,159, US-2004-092688A and US-2004-234607A, there was room for improvement.